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Carbohydrates

Carbohydrates are organic molecules composed of carbon, oxygen and hydrogen.

There are three common types of carbohydrate:

  • Monosaccharides are the simplest carbohydrates. They are sugars.
  • Monosaccharides are small, water-soluble molecules . They are the building blocks (monomers) for larger carbohydrates.

    Glucose, fructose and galactose are common monosaccharides.

  • Disaccharides are formed from two monosaccharides . They are also sugars.
  • Sucrose, lactose and maltose are common disaccharides.

  • Polysaccharides are long chains of monosaccharides . They are not sugars.
  • Polysaccharides are insoluble in water. The structure of polysaccharides can vary:

    Cellulose is a polysaccharide which forms long, straight chains. Starch is a polysaccharide that forms coiled structures. Both are polymers of glucose!

Rice plants store energy in the form of starch in their grains.
Rice plants store energy in the form of starch in their grains.

Carbohydrates have different functions in the cell depending on their size and structure.

Monosaccharides are the main energy source for living organisms. Glucose is the most common energy source.

The breakdown of glucose (called respiration) provides energy for chemical reactions in the cell.

Polysaccharides are insoluble and fairly unreactive. This makes them ideal for structural support and energy storage:

  • Cellulose is formed from long straight chains of glucose molecules. These form tough fibres that are used to make plant cell walls.
  • Starch and glycogen are coiled glucose polymers. This shape packs a lot of glucose into a small space, for energy storage.
Rice plants store energy in the form of starch in their grains.
Rice plants store energy in the form of starch in their grains.

Monosaccharides are classed by how many carbon atoms they have.

Pentose sugars have 5 carbons and hexose sugars have 6 carbons. Both are usually ring shaped.

The carbons are numbered so that we can consistently refer to the sugar structure.

DNA strands are called 3' and 5', referring to the position of the exposed carbon in the sugar backbone.

We number the carbon atoms clockwise from the oxygen. Deoxyribose is a pentose sugar. Glucose is a hexose sugar.
We number the carbon atoms clockwise from the oxygen. Deoxyribose is a pentose sugar. Glucose is a hexose sugar.

Disaccharides are two monosaccharides joined together. All the monosaccharides listed in the table (glucose, fructose and galactose) are hexose sugars.

Disaccharide Monosaccharides Occurrence
Sucrose Glucose and fructose Fruits
Lactose Glucose and galactose Milk
Maltose Two $$\alpha$$-glucoses In germinating seeds

The glycosidic bond is the covalent bond that connects two monosaccharides (e.g. glucose).

The glycosidic bond is formed through a condensation reaction. The two monosaccharides are linked by an oxygen atom.

Recall that a condensation reaction is a reaction in which two molecules combine and a small molecule (usually water) is removed.

Glucose and fructose combine to make sucrose and water.
Glucose and fructose combine to make sucrose and water.

When two monosaccharides form a glycosidic bond, they become a disaccharide. Two glucose molecules form the disaccharide maltose while a glucose and fructose monosaccharide form sucrose.

Glucose is a simple hexose (six carbon atom) sugar. It is a major source of energy within the cell. During respiration, glucose is broken down to produce ATP.

$$\alpha$$-glucose and $$\beta$$- glucose are two isomers of glucose. They differ only in the position of the OH group on their first carbon atom. In one, the OH group points up, and in the other, it points down.

We can remember the position of the OH group using the mnemonic ABBA - alpha below, beta above.

$$\alpha$$-glucose (left) and $$\beta$$-glucose (right). They differ by the position of the $$\ce{-OH}$$ group on $$\ce{C1}$$
$$\alpha$$-glucose (left) and $$\beta$$-glucose (right). They differ by the position of the $$\ce{-OH}$$ group on $$\ce{C1}$$

$$\alpha$$-glucose forms polymers with a curved structure. This is due to the angle of the bond between two glucose molecules. These polymers include starch and glycogen.

$$\beta$$-glucose forms polymers with a linear structure. Alternate glucose molecules must rotate in order to fit together. Cellulose is made of $$\beta$$-glucose.

Cellulose is made of numerous $$\beta$$-glucose molecules. The molecular structure is often represented as shown, with a subscript 'n' to indicate that this unit repeats for the rest of the molecule.
Cellulose is made of numerous $$\beta$$-glucose molecules. The molecular structure is often represented as shown, with a subscript 'n' to indicate that this unit repeats for the rest of the molecule.

Cellulose is a polysaccharide found in plant cells. It is the main component of cell walls.

The monomer (smallest sugar component) of cellulose is $$\beta$$-glucose. Over 1000 monomers are assembled into long straight chains.

The glucose monomers are linked together by 1-4 glycosidic bonds (bonds between the first and the fourth carbon atom of the glucose molecule).

In order to form these bonds, alternate $$\beta$$-glucose monomers must rotate through 180 degrees. This rotation gives cellulose its characteristic straight chained structure.

Cellulose chains are arranged in parallel. These chains are held together by hydrogen bonds and are called microfibrils.

Cellulose is arranged into microfibrils, which give plant cells high tensile strength. This means that they are not easily stretched.

Cellulose walls provide great tensile strength, enabling plant cells to store plenty of water without bursting.
Cellulose walls provide great tensile strength, enabling plant cells to store plenty of water without bursting.

The combination of rigidity and high tensile strength provided by cellulose makes it a good structural molecule.

Cellulose is the most common organic molecule on earth (in terms of total mass). Cotton, for example, is 90% cellulose and a large proportion of leaves and wood is cellulose.

Animals are unable to digest cellulose on their own. This is because animals do not produce cellulase, an enzyme required to digest cellulose.

With the aid of microorganisms that produce cellulase, animals such as cattle (and other ruminants) and termites are able to utilise cellulose as their primary energy source. These microorganisms live in the digestive systems of such animals.

Starch is the major energy storage molecule in plants.

It is formed of two polysaccharides, both made of $$\alpha$$-glucose:

  • Amylose only contains 1-4 glycosidic bonds. It has an unbranched structure.
  • Amylopectin has a branched structure. The $$\alpha$$-glucose monomers are linked by 1-4 and 1-6 glycosidic bonds.

Amylose and amylopectin combine to give starch a spiral structure with some branching. This structure is relevant to its energy storage role:

  • Starch is dense and insoluble. This reduces the space needed for storing starch.
  • The spiral structure with branching makes starch readily accessible to enzymes. This means the starch can be broken down rapidly when energy is needed in the cell.
Amylopectin is branched due to the 1-6 glycosidic bonds. Amylose is unbranched and has only 1-4 glycosidic bonds.
Amylopectin is branched due to the 1-6 glycosidic bonds. Amylose is unbranched and has only 1-4 glycosidic bonds.

Plant cells contain organelles called amyloplasts which convert glucose into starch for storage.

Chitin is a structural carbohydrate found in insects and other arthropods, as well as fungi. It has a high tensile strength.

Crabs are a class of arthropods that have shells made of chitin.
Crabs are a class of arthropods that have shells made of chitin.

Chitin accounts for the rigidity of the exoskeleton (the hard outer skin that supports the soft internal tissue of the organism) in arthropods.

Like cellulose, it is a straight chained carbohydrate. However, it is made up of the monomer N-acetyl glucosamine, which is a derivative of glucose.

Glycogen is often referred to as animal starch. It is the most important carbohydrate used for energy storage in animal cells. It is also found in some fungi, but not in plants.

Glucose monomers at the branch points are connected by 1-6 glycosidic bonds while those in the straight chains are connected by 1-4 glycosidic bonds.
Glucose monomers at the branch points are connected by 1-6 glycosidic bonds while those in the straight chains are connected by 1-4 glycosidic bonds.

Glycogen is similar to amylopectin, which is one of the two components of starch. The monomer in amylopectin is $$\alpha$$-glucose monomers which is linked by 1-4 and 1-6 glycosidic bonds.

Glycogen does not contain amylose (the unbranched component of starch). This makes glycogen more branched than starch.

The higher number of branches makes glycogen less stable and easier to break down. This makes glycogen well-suited for the fast-changing energy demands of animals.

Glycogen is the first source of energy that the body will use up. This source is depleted after just a few hours of fasting.

The largest stores of glycogen are found in the liver. The hormone insulin triggers glycogen production and storage. The hormone glucagon triggers the breakdown of glycogen to glucose.

Benedict's test is a test for simple sugars (e.g. glucose).

  • Grind up your sample and mix in a test tube of distilled water.
  • Add 2 cm$$^3$$ of Benedict's solution ( light blue) to the same amount of your sample.
  • Boil the solution for 2-3 minutes.

If the solution turns a brick-red colour, your sample contains glucose!

Iodine test: tests for starch.

  • Grind up your sample and mix in a test tube of distilled water.
  • Add a few drops of iodine (light brown solution) into the test tube.

If the solution turns a blue-black colour, your sample contains starch!

The presence of starch in a leaf is a good indicator of photosynthesis.